Erythroid cell development into two morphologically and biochemically distinct populations in the chick embryo will continue to be studied. We are examining the control of hemoglobin synthesis both at the level of the transcription of specific classes of globin messenger RNA and at post-transcriptional levels. This project makes use of newly devised methods that allow the erythropoiesis of avian embryonic cells to occur in cell culture. The influence of environmental factors in the control of erythroid cell differentiation and proliferation will be evaluated.